1. Field of the Invention
The invention concerns transmission systems on digital links of frequency division multiplex analog telephone signals (FDM signals), that is analog signals obtained by modulation of carriers having regularly spaced apart frequency using telephone signals whose frequency bandwidth has been limited.
2. Description of the Prior Art
The transmission of FDM signals on a digital link naturally implies their digital coding at the transmission channel input and their decoding at the reception channel output. It has already been proposed (see for example "IEEE TRANSACTIONS ON COMMUNICATIONS", Vol. COM. 25- No. 1, February 1977, Andrews et al., pages 264-271) to transmit FDM telephone signals by pulse code modulation (PCM) non linear coding. It will be shown further on that this coding method cannot provide maximum loading of the digital channel when the FDM phone channels are organized into groups each consisting of a large number of channels.
Recommendations of the CCITT ("Comite Consultatif International Telegraphique et Telephonique") are relative to FDM and particularly those FDM signals whose carriers are spaced apart by 4 kHz and modulated in SSB (single sideband) mode by phone signals which have been limited to the 300-3400 Hz band. They recommend regrouping the above-mentioned channels into a whole organized in hierarchical levels into the following groups:
______________________________________ group: 12 channels in the 60-108 kHz band, supergroup: 60 channels in the 312-552 kHz band, master group: 300 channels in the 812-2044 kHz band, super master group: 900 channels in the 8516-12388 kHz band. ______________________________________
To justify the coding procedure choice adopted for the invention's system, let us briefly consider the application of the various procedures which can be used for coding a SMG (super master group). As we have seen, a SMG is composed of 900 phone channels in a bandwidth of 3872 kHz (limit frequencies 8516 and 12 388 kHz). Once the SMG is brought into the appropriate band, its sampling frequency should then be more than double the band width, that is 7744 kHz. Taking into consideration the frequency margins to be planned for the filtering before coding and after decoding, this sampling frequency should be from 8 to 8.5 MHz. We are going to calculate how many SMGs may be transmitted using the various procedures within a hierarchical digital rate of 140 Mbit/s (exactly 139.264 kbit/s), originally planned for the transmission of 1920 digital channels at 64 kbit/s.
In linear coding, it can be demonstrated that the necessity of limiting the white noise of theoretical quantization brought to the channel to an acceptable level, implies assigning 10 bits to each sample. The digital rate necessary is 80 to 85 Mbit/s while the available rate is 140 Mbits/s. So, only one SMG can be transmitted. To better utilize the remaining digital rate, we can consider assigning it to other transmission purposes but this approach complicates the operation.
Non-linear coding will not be superior to linear coding unless it makes it possible to transmit two SMGs in the available digital rate capacity. So, a compression procedure should be considered with a non-linear characteristic to bring the number of bits per sample to 8. The calculation demonstrates that for the conventional load Cc, the theoretical quantization noise per channel is 800 pWop and consequently remains very considerable.
Coding with a non-integer number of bits, in the case of the SMGs consists of coding the signal into 9-bit sample words according to a non-linear characteristic (for example, a 5 segment characteristic) but only transmitting 8 bits p/n times and transmitting the 9 bits (n-p)/n times. In this way, we obtain an average number of bits per sample of (9-p/n) bits which is no longer an integer. The calculation shows that the improvement of the signal to noise ratio increases as p approaches n, but that it remains insufficient (2 dB only) for realistic values of (n-p)/n, that is for values which remain less than 1/2.
In fact, in the case under consideration, adaptative block coding is the best approach. Computer simulation demonstrates, for a constant throughput rate, that that quantization noise is considerably reduced (for example, by 2.5 dB) in comparison with the quantization noise brought about by non-linear coding with non-integer number of bits. At low levels, this reduction reaches 4 dB. It is to be noticed that adaptative block coding consists of forming sample blocks (that is, series having a constant number of samples), selecting a coding characteristic for each of these blocks from among a determined number of characteristics, and transmitting to the decoder, together with each block of samples, a signal indicating the characteristic chosen for the block to which the samples belong. When transmitting FDM signals, it is advantageous, for the coding of each block, to select the characteristic which provides the minimum quantization noise. The number of characteristics available is limited by the number of bits which can be transmitted by the remaining throughout capacity, that is, the capacity which has not been used by the transmission of sample bits and sync. bits (frame, multiframe, etc-). For blocks of m samples and a number 2.sup..alpha. of characteristics, the throughput rate required for transmitting the signal identifying the characteristic is evidently .alpha./m bits per sample.
The invention's system is not only applicable to the transmission of two CCITT super master groups on a 140 Mbit/s digital channel. It also makes it possible, for example, to transmit on the said 140 Mbit/s channel, two sets of 15 and even 16 supergroups. It also makes it possible to transmit a group of 600 channels on a digital link of 44 736 kbit/s under better conditions than in the system described in the above-mentioned reference of Andrews et al.